#include<iostream>
#include<iomanip>
#include<cmath>
using namespace std;
int main()
{
	int n;
	while (cin >> n && n)
	{
		double arr[110];
		double brr[110];
		for (int i = 0; i < n; i++)
		{
			/*scanf_s("%lf", &arr[i]);
			scanf_s("%lf", &brr[i]);*/
			cin >> arr[i];
			getchar();
			cin >> brr[i];
		}
		arr[n] = arr[0];
		brr[n] = brr[0];
		double sum = 0;
		for (int i = 0; i < n; i++)
		{
			sum += abs(arr[i + 1] * brr[i] - arr[i] * brr[i + 1]);
		}
		cout << fixed <<setprecision(1) << sum / 2 << endl;
	}
	return 0;
}